1. Field of the Invention
The present invention relates to an anti-lock control apparatus for a motor vehicle and particularly to the art of controlling the difference between respective braking torques applied to a right and a left wheel of the vehicle.
2. Related Art Statement
It is widely practiced to provide a motor vehicle with an anti-lock control apparatus for not only shortening the braking distance needed to brake and stop the vehicle running on a road surface but also improving the stability of the running direction of the vehicle when the vehicle is braked. Generally, an anti-lock control apparatus controls respective braking torques applied to a left and a right wheel of a vehicle and thereby prevents each wheel from being locked on a road surface.
An example of the anti-lock control apparatus is disclosed in Japanese Patent Application laid open for inspection under Publication No. 62(1987)-275870. The prior apparatus can perform (a) a normal control in which respective braking torques applied to two front (left and right) wheels of a four-wheel vehicle are controlled, independent of each other, based on respective lock tendencies of the two wheels, and (b) a low-select control in which the respective braking torques applied to the two front wheels are controlled in a common manner based on the higher one of the respective lock tendencies of the two wheels. More specifically described, if an anti-lock control is started when the running speed of the vehicle is not lower than a reference value, the low-select control is selected commonly for the two front wheels and is performed for a predetermined initial period of the anti-lock control and subsequently the normal control is selected and performed for each of the two front wheels, independent of each other.
The prior apparatus performs the low-select control commonly for the two front wheels, when the above-indicated initiating conditions are satisfied, irrespective of whether the deceleration of the vehicle is high or low when the vehicle is braked. However, the degree of necessity to perform the low-select control for the front wheels changes depending upon the vehicle deceleration.
When a four-wheel vehicle is subjected to a deceleration, the vehicle is subjected to a forward load movement, so that vertical loads exerted to front wheels of the vehicle increase and vertical loads exerted to rear wheels of the vehicle decrease. Consequently lateral forces exerted to respective tires of the rear wheels decrease. The amount of forward load movement increases as the vehicle deceleration increases. The vehicle deceleration increases as friction coefficient, .mu., of road surface on which the vehicle is running increases. As shown in FIG. 6, when a difference is produced between respective braking torques applied to the front left and right wheels, the vehicle is subjected to a yawing moment, M, due to the torque difference. The change of the running direction of the vehicle due to the yawing moment M cannot be avoided unless the lateral forces which can stand the yawing moment M are exerted to the tires of the rear wheels.
However, when the vehicle deceleration becomes high because the vehicle is strongly braked on a road surface having a high friction coefficient .mu., the amount of forward load movement becomes large, so that the vertical loads exerted to the rear wheels become low and the lateral forces exerted to the rear wheels also become low. In this case, if the above-identified prior apparatus selects and performs the low-select control for decreasing the difference between the respective braking torques applied to the front left and right wheels, the yawing moment M due to the torque difference becomes low. Consequently the stability of the running direction of the vehicle is maintained in spite of the low lateral forces exerted to the tires of the rear wheels. Thus, when the vehicle deceleration is high, the necessity to maintain the stability of the vehicle's running direction is high and the necessity to perform the front-wheel low-select control is also high.
In contrast, when the vehicle deceleration remains low because the vehicle is braked on a road surface having a low friction coefficient .mu., the amount of forward load movement is small, so that the vertical loads exerted to the rear wheels remains sufficiently high and the lateral forces exerted to the rear wheels also remains high. In this case, even if the above-identified prior apparatus does not perform the low-select control for the front wheels, the stability of the running direction of the vehicle is maintained. Accordingly, there remains a possibility that the respective braking torques applied to the front wheels be increased for shortening the braking distance. Thus, when the vehicle deceleration is low, the necessity to maintain the stability of the vehicle's running direction is lower than when the vehicle deceleration is high, therefore the necessity to perform the front-wheel low-select control is also lower.
While the above discussion is made for the low-select control performed for the front left and right wheels, the following discussion is made for the low-select control performed for the rear left and right wheels:
Generally, when the low-select control is performed for the rear left and right wheels, the slip ratios of the rear wheels becomes lower than when the normal control is performed, and the lateral forces exerted to the tires of the rear wheels become high. In contrast, when the normal control is performed for the rear wheels, the slip ratios of the rear wheels becomes higher than when the low-select control is performed, and the braking forces applied to the rear wheels become high. Therefore, when the vehicle deceleration is high because the vehicle is braked on a road surface having a high friction coefficient .mu. and accordingly the vertical loads exerted to the rear wheels become low, it is advantageous for the prior apparatus to select and perform the low-select control for the rear wheels, for maintaining the stability of the running direction of the vehicle. In contrast, when the vehicle deceleration is low because the vehicle is braked on a road surface having a low friction coefficient .mu. and the vertical loads exerted to the rear wheels remains sufficiently high, it is advantageous for the prior apparatus to select and perform the normal control for the rear wheels, for shortening the braking distance.
Thus, irrespective of whichever the low-select control is performed for, the front wheels or the rear wheels, it is advantageous for the prior apparatus to select and perform the low-select control when the vehicle deceleration is high, and not to select the low-select control when the vehicle deceleration is low, for the purpose of improving its vehicle-braking performance including shortening the vehicle-braking distance and maintaining the stability of vehicle's running direction.